1. Field of the Invention
The present invention relates to a diesel engine, and more particularly to a diesel engine in which an early-stage injection that is performed at an earlier stage than normal pilot injection, and a main injection that is performed thereafter.
2. Description of the Related Art
Conventionally, in diesel engines, it is known that a pilot injection and a main injection are performed. In such engines, a relatively small amount of pilot injection is first performed at a timing in the vicinity of compression top dead center (generally, around 10° BTDC to 10° ATDC), and a relatively large amount of main injection is performed immediately after this pilot injection. The fuel provided by the pilot injection is ignited immediately following injection; this acts as a flame seed that aids the ignition of the main injection. Since the ignition lag of the fuel injected in the main injections is suppressed, and rapid early-stage combustion and an abrupt increase in the pressure inside the cylinder are suppressed, vibration noise is suppressed.
However, in this pilot and main injection, since the main-injection fuel suddenly enters the combustion chamber in which combustion caused by the pilot injection, the drawback of smoke generation is encountered. Furthermore, the drawback of the generation of NOx by the combustion of the pilot-injection fuel is also encountered. Since there is a flame seed created by the pilot injection, the problem of an aggravation of NOx due to the combustion of the main-injection fuel does not arise.
In recent years, meanwhile, in order to achieve a simultaneous reduction in smoke and NOx, uniform pre-mixed combustion has been proposed in which fuel injection is performed using a timing (in concrete terms, during the compression stroke before pilot injection) and amount which are such that ignition does not occur immediately following fuel injection, a pre-mixture is formed inside the cylinder, and this pre-mixture is ignited in the vicinity of compression top dead center. In this combustion method, fuel injection is generally performed once, without being divided into pilot and main injection.
However, in this combustion method, since fuel injection is performed in a state in which the interior of the cylinder is at a low pressure and the air density is low, i.e., during the transition from the intake stroke to the compression stroke, the penetration of the fuel jet mist is relatively strong, so that the fuel jet mist directly collides with and adheres to the inside walls of the cylinder. As a result, problems such as the exhaust of unburned fuel (HC), an aggravation of CO accompanying incomplete combustion and the like arise. Furthermore, in cases where it is attempted to create a completely uniform pre-mixture inside the cylinder, it is desirable to perform fuel injection at as early a stage as possible, e.g., during the intake stroke; if this is done, however, it becomes difficult to cause auto-ignition of the pre-mixture at the desired timing, so that the combustion period tends to become unstable.
Accordingly, in recent years, a combustion method has been proposed in which such pre-mixture combustion is divided into two injections, i.e., an early-stage injection and a main injection, a pre-mixture is formed by the early-stage injection during the compression stroke, the main injection is performed in the vicinity of compression top dead center, and the ignition period is thus controlled.
Even in this case, however, various problems such as the problem of adhesion of the fuel injected by the early-stage injection to the inside walls of the cylinder, an increase in the internal temperature of the cylinder by the fuel injected in the early-stage injection and the like arise; accordingly, it is difficult to effect favorable combustion without a deterioration in the exhaust gas. In the case of a single early-stage injection, it is difficult to cause uniform mixing of the fuel jet mist and air in the time before compression top dead center is reached, so that a region where the fuel is concentrated (rich) remains inside the cylinder, thus causing an oxidation reaction of the fuel to occur in the stage before compression top dead center is reached; this leads to a rise in the temperature of the combustion chamber, and also caused a deterioration in the fuel economy. Furthermore, in cases where the main injection is performed in the vicinity of compression top dead center following this early-stage injection, the main-injection fuel is burned at once, so that the problem of NOx is aggravated. It is also conceivable that the main injection timing might be retarded in order to avoid NOx; if this is done, however, there is an aggravation of HC and CO. Accordingly, in the case of a pre-mixture combustion method using this early-stage injection and main injection, it is difficult to obtain the intrinsic merits of pre-mixed combustion.